Applications of Physics to Archery
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Applications of Physics to Archery H. O. Meyer Physics Department, Indiana University [email protected] (6 November 2015) . Abstract: Archery lends itself to scientific analysis. In this paper we discuss physics laws that relate to the mechanics of bow and arrow, to the shooting process and to the flight of the arrow. In parallel, we describe experiments that address these laws. The detailed results of these measurements, performed with a specific bow and arrow, provide insight into many aspects of archery and illustrate the importance of quantitative information in the scientific process. Most of the proposed experiments use only modest tools and can be carried out by archers with their own equipment. 1 INTRODUCTION For more than 10,000 years, human civilizations relied on bow and arrow to provide food and to fight wars. As gunpowder gradually displaced human-powered weapons, archery declined until the 18th century when it experienced a revival as a recreational activity and as a modern sport. The behavior of bows and arrows, the shooting process, and the flight of the arrow towards the target are described and explained to a large extent by physics, mainly mechanics, elasticity and aerodynamics. Recognizing this, bowmen with scientific knowledge began to perform quantitative experiments with their bows around 1920 [1,2]. An anthology of early scientific archery papers was published as a book in 1947 [3]. Insight gained from these studies questioned the traditional longbow design and transformed bow making from a craft to a science. The continuing advancement of archery equipment, based on scientific principles, has resulted in the modern Olympic recurve bow and in the compound bow, which uses a system of cables and pulleys to modify the draw force. Crucial improvements are also due to the emergence of new plastics and compound materials, replacing traditional ingredients, such as wood, linen and animal hide. This paper, written for scientists with an interest in archery, contains a discussion of physics laws that apply to various aspects of archery and a description of experiments to test these laws. Most of these measurements require only modest tools and can be performed by readers using their own equipment. In the context of this paper, data from the proposed experiments are 1 collected for a specific bow (a compound bow) and a specific arrow, demonstrating how the understanding of many aspects of archery requires quantitative information. 2 THE ARROW 2.1 Shape, straightness and mass A modern archery arrow is shown in Fig. 1. It consists of four parts: (1) a shaft made from tubular carbon-fiber compound, aluminum, or a combination thereof, (2) the tip, or ‘pile’ made from steel or brass, sometimes screwed into an aluminum insert, (3) fletching, consisting of three or four fins of a variety of materials, shapes and sizes, and (4) a nock at the rear end of the arrow, which clips onto the bowstring and is typically made of plastic and often mounted in an aluminum insert as well. By convention, the length L of an arrow is defined as the distance from the nocking point (where the bowstring touches the arrow) to the front end of the shaft, excluding the tip. We assume that the arrow is symmetric around the z-axis of a Cartesian frame. The nocking point fixes z = 0. The y-axis shall be up and the x-axis sideways. The shaft of the arrow consists of a hollow cylinder with an outer radius R and wall thickness ΔR. The straightness of today’s carbon arrows is excellent. For instance, the axis of a moderately-priced carbon arrow is guaranteed to deviate by less than 100 μm (about the thickness of human hair) from a straight line. One can test this by rotating an arrow resting on V- notches at both ends. The wobble amplitude of the shaft in the middle, observed with a microscope, equals the deviation from straight. We have measured 10 of our sample arrows and found deviations ranging from 10 to 100 μm, with an average of 55 μm, and an accuracy of about 5 μm. This is nice to know, but it is not clear if and how such a small deviation from straight will affect the trajectory of an arrow, especially when this arrow is rotating and oscillating. A measurement of the mass M of the arrow and that of its components requires a scale with a precision of ± 0.01 g. The distribution of the mass along the sample arrow, evaluated by weighing all parts separately, is shown in Fig. 1. From these data, the location zcm of the center of mass can be calculated. It agrees with the center of mass found by balancing the arrow on an edge. A quantity called ‘FOC’ is widely used to quantify the amount by which the center of mass is ‘Forward-Of-Center’, defined as z FOC ≡−cm 1 . (1) L 2 It is generally accepted by the archer’s community that the FOC value should range from 0.07 to 0.17, but quantitative evidence backing up this belief seems to be lacking (see the tests with varying FOC, reported by Hickman in Ref. 3, p.77) 2 Fig. 1 Linear mass distribution of the sample arrow [4]. The four parts of an arrow are indicated by numbers (see text) Table 1. Static parameters of the sample arrow [4] L 0.696±0.001 m arrow length R 3.63±0.05 mm shaft radius ΔR 0.50±0.07 mm wall thickness M 20.62±0.01 g total arrow mass mshaft 11.70±0.01 g (57% of total) mtip 6.20±0.01 g (30%) mfins 1.41±0.01 g (7%) mnock 1.24±0.01 g (6%) zcm 0.414±0.001 m center of mass coordinate FOC 0.093±0.002 -- ‘Forward-Of-Center’ S 5.17±0.03 Nm2 stiffness Spine 492±3 -- ATA ‘spine’ 2.2 Stiffness The stiffness S is a property of the shaft material and quantifies the ability of an arrow to bend. It is of crucial importance in archery for two reasons. First, the stiffness must have a minimum value to prevent destruction of the arrow during acceleration (sects. 4.1, 4.2), and second, it has to have a specific value for the proper interaction between the bow and the arrow during the shooting process (sect. 4.3). The stiffness is easily measured as follows. In the setup shown in Fig. 2, a section of the arrow is supported by two knife edges, which allow the arrow to tilt around the x-direction. Small notches in the edges keep the arrow from rolling off sideways. The distance Δz between the two supports is arbitrary, but must be known (here, Δz = 0.550 m). 3 Fig. 2 Setup to measure the stiffness of the sample arrow A varying downward force Fb is applied halfway between the supports by hanging weights or by pulling with a luggage scale. The downward deflection ym at the mid-point is measured with a dial indicator. The measured ym as a function of Fb are shown in Fig. 3. As expected for elastic deformation, the deflection is proportional to the applied force,. The physics that governs the slope ym/Fb is understood within the Euler-Bernoulli beam theory [5] which is valid for small deflections of thin beams under lateral loads. The expected result for the present situation is Δz3 yF≡ . (2) mb48S This equation defines the stiffness S [Nm2]. Since all other quantities in Eq. (2) have been measured, the stiffness can be deduced (listed in table 1). The Archery Trade Association (ATA) proposes to measure stiffness by a different quantity, called Spine which is in common use. Their definition reads: “Spine is the number of thousands of an inch by which the center of an arrow shaft of 28 inch length is displaced when a sideways force of 1.94 pounds is acting at that point”. To convert stiffness S into Spine, the above definition, expressed in SI units, is inserted into Eq. 2. It follows that “Spine equals 2544 divided by the stiffness S in Nm2”. The result for our sample arrow is 492, close to the value of Spine 500, quoted by the manufacturer. 4 20 15 (mm) m y 10 5 deflection 0 0 102030 center force Fb (N) Fig. 3 Deflection ym versus applied force Fb, measured with the setup shown in Fig.2 It turns out that stiffness is the product of two factors, one related to the geometry and the other to the material properties of the shaft. =⋅ SJYxxz. (3) The geometry factor, Jxx, is the second moment of the beam cross-section relative to the x- axis. For a hollow cylinder with an outer radius of R and inner radius of R−ΔR, Jxx is given by π J==−−Δ y244 dx dy() R() R R . (4) xx 4 ─11 4 With R and ΔR from table 1, one obtains Jxx = 6.10·10 m . When we assume that ΔR is much 3 smaller than R, the moment Jxx (and thus the stiffness) is proportional to ΔR and to R . The stiffness of an arrow is therefore affected more strongly by the diameter than by the wall thickness. The other factor in Eq. (3) is the elastic modulus, which quantifies strain in response to a given stress and is a material property. The mechanical properties of carbon fiber composite materials are not well defined and even depend on direction (here we need the modulus Yz that applies in the z-direction).